The present invention generally relates to blood filtration systems and methods. More particularly, the invention relates to a system for continuously filtering a blood product passed through the system, and to a method of use of the system.
Most of the whole blood collected from donors today is not typically stored as to whole blood and used for transfusions. Instead, the blood is first separated into its components, including erythrocytes (red blood cells), leukocytes (white blood cells), platelets and plasma, and the components are then individually stored and used to treat specific diseases and disease states. Erythrocytes, for example, are used to treat anemia. Platelets are used to control diseases such as thrombocytopenic bleeding, and plasma can be used as either a volume expander, or as a source for clotting factor VIII for use in the treatment of hemophilia.
Blood collection systems in the art typically include multiple plastic bags interconnected by multiple plastic tubes. Such systems include nonsterile systems that are open to the atmosphere, and sterile systems that are closed to communication with the atmosphere. Open collection systems are subject to government regulation that controls the plastic materials that are used to fabricate the multiple interconnected blood bags. They are also subject to government regulations that limit the maximum storage periods for blood components collected using these systems.
Closed blood collection systems permit blood storage for extended periods of time and have gained wider acceptance. Erythrocytes, for example, collected can be stored for up to forty-two days, depending on the type of anticoagulant and storage container used. Platelets can be stored for up to five days, depending on the type of storage container used. Plasma may be frozen and stored for even longer periods of time. Closed storage systems are also more reliable because the systems provide a more sterile environment that maximizes the possible storage period and minimizes the presence of impurities or other materials that may cause undesired side effects to recipients.
Closed blood collection systems known in the art typically include a number of sealed blood compatible bags that are connected together by blood compatible conduits, and include a filter disposed on a conduit between any two bags. Blood component collection according to these systems is accomplished by collecting whole blood from a donor or a blood source in a first of these bags and separating the blood into its components by squeezing the blood from the first bag through the conduit and filter into a second bag. This method of filtration is repeated until the desired filtered blood product is attained. Such blood collection systems are exemplified by U.S. Pat. No. 3,986,506 issued to Garber, et al., U.S. Pat. No. 4,596,657 issued to Wisdom, and U.S. Pat. No. 5,527,472 issued to Bellotti, et al. A similarly configured closed blood collection system that utilizes the force due to gravity to accomplish the steps of separation by filtration is exemplified by U.S. Pat. No. 6,059,968 issued to Wolf, Jr. It is unknown whether such systems are, indeed, effective in accomplishing the stated filtration objectives. Regardless, however, such systems are undesirable as they fail to permit or enable the continuous filtration of blood to produce the desired blood product. They are also complex in construction, cumbersome to use, and are typically expensive to produce.
FIG. 1 illustrates one type of continuous filtration closed blood collection system which utilizes multi-bag collection concept. The system shown in FIG. 1 is the model 994CF-blood collection system manufactured by Haemonetics Corporation of Braintree, Mass., USA. The 994CF system is adapted for filtering leukocytes from platelet rich plasma to produce a purer or more concentrated platelet product. The platelet rich plasma is typically provided to the 994CF system in cycles during apheresis, in which whole blood is drawn from a donor and separated into its constituent components such that one or more components are collected while the remaining components are returned to the donor. As shown in FIG. 1, the 994CF system 100 includes a reservoir bag 102, a platelet collection bag 104, a filter 106, and a conduit 108 that connects the reservoir bag 102 to the platelet collection bag 104.
The reservoir bag 102 includes a top or “chimney” port 114 for receiving the platelet rich plasma through a conduit 116, a channel 110 vertically disposed from a midpoint of the bag to its bottom, and an outlet port 112 located at the bottom of the channel 110. Filter 106 is a leukoreduction filter, which is positioned on the conduit 108 between the reservoir bag 102 and the platelet collection bag 104. Leukoreduction filters have two qualities that filtration methods must handle. First, once the filter 106 is primed to (wetted for the first time), it will no longer pass air. Any air that is introduced into the filter 106 will become lodged against the filtering membrane, and effectively reduce the filtering capacity of the filter 106. Large amounts of air will clog the filter 106 entirely and stop flow. Second, the flow rate of the solution to be filtered affects the efficiency of the filter 106. The channel 110 has a volume capacity of approximately 15 ml. Typically, both the platelet collection bags 104 and the reservoir bag 102 are hung on the same IV pole.
At the beginning of the first platelet collection cycle with the 994CF system 100, the conduit 108, the reservoir bag 102, the leukoreduction filter 106 and the platelet collection bags 104 are empty and dry. As the first platelets are expressed into the reservoir bag 102, a rivulet of fluid travels down the vertical channel 110 and into the leukoreduction filter 106. The volume of fluid in the rivulet is insufficient to fill the vertical channel 110. After several seconds, the inherent resistance of the filter 106 causes the fluid to back up into the vertical channel 110. Fluid fills the vertical channel 110, causing a fluid height difference between the full vertical channel 110 and the empty platelet collection bag 104. Gravity applies a force to the fluid until the fluid level in the vertical channel 110 equals the fluid level in the platelet collection bag 104. As the reservoir bag 102 and the platelet collection bag 104 are typically hung on a single IV pole at similar heights, there is always some fluid held back in the vertical channel 110. This prevents any air from being introduced into the filter 106 in subsequent cycles. The narrow dimension of the vertical channel 110 ensures that there is initially a fluid height difference between the reservoir bag 102 and the platelet collection bag 104 in every cycle.
The platelet collection process during apheresis is discontinuous. Each cycle produces between 20 and 70 ml of platelet product that is expressed through the conduit 108 and deposited into the reservoir bag 102. The delay between subsequent platelet collection cycles is typically 12 to 15 minutes. After the final platelet collection cycle, the vertical channel 110 holds approximately 10 to 15 ml of fluid. Thereafter, the operator removes the platelet collection bag 104 from the IV pole, and lowers it relative to the reservoir bag 102 to allow the volume of fluid remaining in the channel 110 to be filtered.
Though more advantageous than other multi-bag blood collection systems in the art, the 994CF system 100 also presents disadvantages. During the first cycle, for example, the rivulet of fluid introduced into the reservoir bag is insufficient to fill the vertical channel 110. This allows a mix of fluid and air to enter the filter 106, potentially clogging a portion of the filter 106. The structure of the reservoir bag also buffers or reduces the pressure (priming pressure) of fluid flowing to the dry filter. The reduced fluid pressure renders it more difficult to ensure that the entire filter is fully wetted, or primed at the outset. After the final cycle, the flow rate of the fluid through the filter 106 depends on the rate at which the operator lowers the platelet collection bag 104. Processing the final 10 to 15 ml of fluid through the filter 106 at a high rate can dislodge white blood cells from the filter 106. In addition, the chimney port 114 of the reservoir bag 102 is difficult to package efficiently, as kinks may form where the conduit 116 is bonded to the chimney port 114. Moreover, production of the reservoir bag 102 is more expensive than the production of a typical bag as the chimney port 114 requires additional manufacturing steps.